The present invention relates to a counter apparatus for counting radioactive rays and, more particularly, a counter apparatus with a correction device for correcting the non-linearity of the count rate characteristic of the counter apparatus.
By convention, in case of the heart diagnosis in nuclear medicine, a radioisotope is injected into a patient's blood and radioactive rays, for example, gamma rays, produced by the nuclear decay of the radioisotope are measured by a scintillation camera, for example. In this case, the pulse rate of the gamma rays supplied to a detector of the scintillation camera per unit time generally does not coincide with the count rate of the counter apparatus at the final stage of display. That is, the count rate of the counter apparatus should be a linear function of the input pulse rate as shown in a straight line indicated by reference character A in FIG. 1. However, the count rate actually changes curvilinearly as indicated by reference character r. The reason for this difference is that, while radioactive pulses due to gamma rays are produced substantially in a Poisson distribution, the scintillation camera fails to count some pulses as a result of its proper dead time. The number of counts omitted increases as the input pulse rate increases.
As will be easily understood, when the relation of the count rate to the input pulse rate is not linear, data obtained are distorted, thus resulting in incorrect observations of the patient's heart.
Therefore, it is desirable for the scintillation camera to count linearly with regard to the input pulse rate at least within a practical use range. For example, in the conventional scintillation camera, if the dead time is 5 .mu.s and the input pulse rate is 25 kcps (kilocounts per second), the actual count rate curve deviates approximately 10% from the ideal count rate characteristic curve, i.e. linear characteristic.
One method used to correct this non-linear relationship so as to produce the ideal linear curve, the data obtained by the scintillation camera, i.e. those displayed in the display device, are inputted to a computer and processed by using a specific software. For example, in the computer, all the counting data are stored and thus collected data are multiplexed by proper correcting coefficients to add the count-missed value to the counting value.
However, this method is disadvantageous in that it fails to correct the non-linearity of the count rate in real time.